1. Field of the Invention
This invention relates generally to Darrieus-type vertical axis wind turbines, and relates more particularly to a vertical axis wind turbine having means for tensioning the blades of the wind turbine to resist wind-induced buckling loads when parked and to avoid or dampen dynamic resonances when rotating.
2. Description of the Relevant Art
A Darrieus-type vertical axis wind turbine ("VAWT") typically has two curved blades joined at the ends to the top and bottom of a rotatable, vertical tower. The blades bulge outward to a maximum diameter about midway between the blade root attachments points at the top and bottom of the tower. See U.S. Pat. No. 1,835,018 to D. J. M. Darrieus for a basic explanation of such a VAWT. The rotatable, vertical tower with the blades attached will be referred to herein as a rotor or rotor assembly. A typical VAWT supports the bottom of the rotor on a lower bearing assembly, which in turn is elevated off the ground by a base. The rotation of the rotor is coupled to and drives an electrical generator, typically located in the base, that produces electrical power as the rotor rotates. The top of the rotor is supported by an upper bearing assembly that is held in place by guy wires or other structures. FIG. 1 of the drawings is an illustration of a typical prior art VAWT.
A key component of the VAWT are the blades, which interact with the wind to create lift forces that rotate the rotor and drive the generator. The blades typically have a symmetrical or semi-symmetrical airfoil shape in cross-section with a straight chord that is oriented tangential to the swept area of the turbine. The rotor rotates faster than the wind, and the wind generates lift forces on the blades that maintain rotation of the rotor. The lift forces are periodic because each blade goes through two phases of no lift per revolution when the blade is moving either straight up-wind or straight down-wind. In addition to the wind-generated lift forces, centrifugal forces also act on the blades.
A slender structure like a VAWT blade attached by its ends to a rotating axis tends to take the shape of a troposkein when the rotor rotates. A troposkein is the shape that a linearly-distributed mass like a skipping rope would take under centrifugal force when the rope is spun around an axis. Considering just centrifugal forces, the spinning rope takes the troposkein shape and is loaded in pure tension because it has negligible stiffness or resistance to bending. It is desirable for a VAWT blade to have a troposkein shape in order to minimize bending stresses and fatigue loads, but a practical problem is how to design a VAWT blade so that it is flexible enough to assume a troposkein shape yet rigid enough to withstand operating loads, including the significant loads that result from gravity.
In high velocity winds or wind gusts associated with storms, the winds may produce excessively high loads on the VAWT, in which case the rotor must be stopped and parked. When a VAWT is parked, the centrifugal force that maintains the troposkein shape of the blade is obviously not present, and the blade that is upwind is subject to wind loads that tend to buckle the blade inward toward the tower. In order to avoid structural damage, the blades must be strong enough to resist these buckling loads. The stiffness of a blade is directly related to its cross-sectional area; increasing the cross-sectional area of a blade will increase its stiffness and improve its resistance to buckling. Increasing the cross-sectional area of a blade, however, will detrimentally affect performance because of greater blade weight and aerodynamic drag and will also increase cost.
This tradeoff between buckling stiffness and performance is a significant factor in scaling-up the size of a VAWT, and may prevent a larger VAWT from achieving the same efficiencies as a smaller VAWT, or even being economical at all. Furthermore, the weight increase in the blades associated with increased stiffness also causes increased loads on other components, such as the tower and blade attachment structures, which must be increased in size and weight (and cost) in order to compensate.
Adequate buckling stiffness is also a constraint in designing VAWT's with higher height-to-diameter ratios ("H/D"), which may be advantageous to optimize energy recovery from a wind site, especially when coupled with an increased height. A VAWT with a relatively low H/D has blades that bulge outward to a greater degree than a VAWT having a higher H/D. The greater outward bulge of the low H/D VAWT imparts greater compressive strength and resistance to buckling than a relatively flat blade profile of the higher H/D VAWT, all else being equal. Therefore, increasing the H/D ratio weakens the buckling stiffness of a blade.
Another problem with scaling-up the size of a VAWT is that of resonant frequencies. Ideally, the VAWT operates at a frequency that is less than the lowest resonant frequency of the VAWT and its major components. By scaling-up the size of a VAWT, however, the larger and heavier structures of a larger VAWT will tend to have lower resonant frequencies than that of a smaller VAWT. If the resonant frequency of any component of a VAWT is within the operating frequency of the VAWT, then the structure will be subjected to potentially destructive resonant loads.
The competing design constraints of blade buckling and resonant frequencies make the design of larger VAWT's very difficult.